1. Field of the Invention
This invention relates to an OFDM receiving device and also to an OFDM receiving method to be used for digital broadcasting by orthogonal frequency division multiplexing (OFDM).
2. Related Background Art
Various modulation techniques referred as orthogonal frequency division multiplexing (OFDM) have been proposed in recent years for broadcasting digital signals. With an OFDM system, a transmission band is provided with a number of orthogonally arranged sub-carriers and data are assigned to the amplitude and the phase of each sub-carrier for the purpose of digital modulation by PSK (phase shift keying) or QAM (quadrature amplitude modulation).
With OFDM, while each sub-carrier has a small bandwidth because the transmission band is divided into a number of sub-carriers and hence the modulation speed per sub-carrier is low, the overall transmission rate remains practically same as that of any conventional modulation system. Additionally, OFDM is characterized by a low symbol rate also due to the fact that a number of sub-carriers are used in parallel for signal transmission. Therefore, with OFDM, the time length of a multi-path can be reduced relative to that of a symbol in order to reduce the possible interference in the multi-path. Furthermore, with OFDM, since data are assigned to a number sub-carriers, the transmission/reception circuit can be configured by using an IFFT (Inverse Fast Fourier Transform) operational circuit for modulation and an FFT (Fast Fourier Transform) operational circuit for demodulation.
Because of the above identified advantages of OFDM, studies have been made to apply it to ground wave digital broadcasting that can be strongly affected by multi-path interference. In Japan, a standard referred to as ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) have been proposed.
Meanwhile, with OFDM, each channel is normally provided with band gaps for preventing interference from the adjacent channels, which are referred to as guard bands and have a predetermined frequency bandwidth as shown in FIG. 1 of the accompanying drawing. However, the provision of guard bands inevitably increases the bandwidth occupied by each channel to consequently reduce the efficiency of frequency exploitation.
In view of this problem, the applicant of the present patent application has proposed in International Publication No. WO 00/52861 a connected transmission method (which is also referred to as concatenated transmission method) for OFDM signals, by which the center frequencies of the OFDM signals in the frequency domains of a plurality of information channels are modified respectively and then the OFDM signals in the frequency domains of the plurality of information channels are multiplexed in the sense of frequency and collectively subjected to inverse Fourier transform.
With the proposed connected transmission method for OFDM signals, when transmitting three streams of information through respective three information channels (Ch1, Ch2, Ch3), the guard bands separating the channels can be removed and the three information channels can be connected in the sense of frequency axis for signal transmission as illustrated in FIG. 2.
An OFDM transmitter adapted to connected transmission of OFDM signals will be described below in greater detail.
FIG. 3 is a schematic block diagram of an OFDM transmitter adapted to connected transmission of OFDM signals. While any number of channels can be connected together with the proposed technique, it is assumed here that three channels (Ch1, Ch2, Ch3) are connected to transmit three streams of information. Also assume that the center frequencies of the information channels in the RF band are respectively f1 for the first channel, f2 for the second channel and f3 for the third channel as shown in FIG. 2.
The OFDM transmitter 101 comprises a first channel encoder 102-1, a second channel encoder 102-2, a third channel encoder 102-3, a first frequency converter section 103-1, a second frequency converter 103-2, a third frequency converter 103-3, a multiplexer 104, an IFFT operational circuit 105, a guard interval adder 106, an orthogonal modulator 107, a frequency converter 108 and an antenna 109.
The first channel encoder 102-1 receives an information stream as input through the first information channel. It is adapted to operate for Reed-Solomon coding, energy dispersion, interleaving, convolutional coding, mapping and configuring an OFDM frame. The first channel encoder 102-1 generates first channel data as an OFDM signal of the frequency domain of the first channel by carrying out the above operations. The center frequency of the first channel data generated as an OFDM signal of the frequency domain output from the first channel is made equal to 0.
The second channel encoder 102-2 and the third channel encoder 102-3 operate like the first channel encoder 102-1 respectively for the information stream of the second information channel and that of the third information channel. Additionally, the center frequency of the OFDM signal of the frequency domain output from the second channel (the second channel data) and that of the OFDM signal of the frequency domain output from the third channel (the third channel data) are also made equal to 0.
The first frequency converter 103-1 performs a processing operation of frequency conversion for shifting the center frequency of the first channel data (the OFDM signal of the corresponding frequency domain) output from the first channel encoder 102-1. More specifically, the first frequency converter 103-1 converts the center frequency of the first channel data from 0 to (f1−f2).
The second frequency converter 103-2 performs a processing operation of frequency conversion for shifting the center frequency of the second channel data (the OFDM signal of the corresponding frequency domain) output from the second channel encoder 102-2. More specifically, the second frequency converter 103-2 converts the center frequency of the second channel data from 0 to (f2−f2).
The third frequency converter 103-3 performs a processing operation of frequency conversion for shifting the center frequency of the third channel data (the OFDM signal of the corresponding frequency domain) output from the third channel encoder 102-3. More specifically, the third frequency converter 103-3 converts the center frequency of the third channel data from 0 to (f3−f2).
It will be appreciated that the second channel data is actually not subjected to frequency conversion because it is located at the center of the three channels connected for data transmission.
The multiplexer 104 multiplexes the channel data output from the first frequency converter 103-1, the second frequency converter 103-2 and the third frequency converter 103-3 in the sense of frequency to generate a multiplexed signal.
The IFFT operational circuit 105 performs an operation of inverse Fourier transform collectively on the multiplexed signals of the three channel data as multiplexed by the multiplexer 104 to generate an OFDM signal of the base band of time domain. As shown in FIG. 4, the frequency characteristics of the generated OFDM signal of the base band are such that the center frequency of the first information channel is (f1−f2), that of the second information channel is 0 and that of the third information channel is (f3−f2). In the OFDM signal of the base band, the pieces of information of the first through third information channels are subjected to frequency division and multiplexing and maintain orthogonality in order to eliminate any inter-code interference among all the carrier waves.
The guard interval adder 106 adds a guard interval to the OFDM signal of the base band from the IFFT operational circuit 105. As shown in FIG. 5, each signal to be transmitted by the OFDM system is actually transmitted on the basis of a unit of symbol referred to as OFDM symbol. An OFDM symbol comprises an effective symbol representing a signal period during which an IFFT operation is performed for transmission and a guard interval where a rear part of the effective symbol is copied. The guard interval is arranged in a front part of the OFDM symbol. The guard interval adder 106 generates such a guard interval and adds it to the effective symbol.
The orthogonal modulator 107 orthogonally modulates the OFDM signal of the base band, to which a guard interval is added, relative to the carrier wave with an intermediate frequency band of frequency fIF and outputs an IF signal.
The frequency converter 108 multiplies the IF signal output from the orthogonal modulator 107 by the carrier wave signal with a frequency of f2+fIF to produce a signal to be transmitted in an RF signal band.
The signal produced by the frequency converter 108 is then transmitted by way of the antenna 109.
Thus, as described above, the OFDM transmitter can carry out a connected transmission of OFDM signals by changing the center frequencies of the channel data of the three information channels (the OFDM signals of the frequency domains), multiplexing them in the sense of frequency and performing an operation of inverse Fourier transform collectively on the OFDM signals of the frequency domains of the information channels.
With such a connected transmission, a single operation of IFFT is performed collectively on the three channels to maintain orthogonality in order to eliminate any inter-code interference among the sub-carriers. As a result, no interference occurs in the connected three channels and therefore the OFDM transmitter 101 can transmit information for three channels without providing guard bands for preventing interferences with adjacent channels.
An OFDM receiver for receiving such a signal is adapted to detect the IF signal by tuning the oscillation frequency of the local oscillator in the center frequency of the desired information channel. For instance, the oscillation frequency of the local oscillator will be tuned in the frequency (f1) for receiving the signal of the first information channel, in the frequency (f2) for receiving the signal of the second information channel and in frequency (f3) for receiving the signal of the third information channel. The detected IF signal is then orthogonally demodulated by means of the carrier wave of the frequency (fIF) and transformed into an OFDM signal of the base band of time domain. It will be appreciated that the center frequency of the OFDM signal of the base band is equal to 0 regardless of the information channel selected for signal reception. Then, the OFDM signal of the base band is subjected to an operation of FFT to obtain the channel data of the OFDM signal of the frequency domain by demodulation.
Thus, if the OFDM signals of the frequency domains of a plurality of information channels are multiplexed in the sense of frequency and subjected collectively to an operation of inverse Fourier transform for connected transmission, the OFDM receiver can selectively receive the signal of only one of the channels by tunning the oscillation frequency of the local oscillator in the center frequency of the desired information channel.
Now, the frame configurations as defined in the ISDB-T Standard (in the case of Mode 1) for a broadcasting mode of digital ground wave broadcasting will be discussed below.
As shown in FIGS. 6 and 7, according to the ISDB-T Standard defines the data structure of an OFDM frame for data to be transmitted. FIG. 6 illustrates the frame configuration to be used for modulating an information signal by differential modulation (differential quadrature phase shift keying—DQPSK) and FIG. 7 shows the frame configuration to be used for modulating an information signal by synchronous modulation (quadrature phase shift keying—QPSK, 16 quadrature amplitude modulation—16QAM, 64 quadrature amplitude modulation—64QAM).
Referring to FIGS. 6 and 7, a total of 108 data (with carrier numbers #0 through #107) are transmitted by a symbol. The unit of data of a symbol is referred to as OFDM symbol. Also note that 204 OFDM symbols (with symbol numbers #0 through #203) constitute an OFDM frame.
An OFDM frame contains information signals (S0,0 through S95,203) that are orthogonally modulated by QPSK, 16QAM or 64QAM along with various control signals such as CP (Continual Pilot) signal, TMCC (Transmission and Multiplexing Configuration Control) signal, AC (Auxiliary Channel) signal and SP (Scattered Pilot) signal.
The CP signal is a signal with a fixed phase and a fixed amplitude. When modulating the information signal by differential modulation, a CP signal is arranged at the leading carrier of each OFDM symbol (at a position where the frequency is lowest). When transmitting the information signal by connected transmission, a CP signal is arranged at the rightmost position of the connected transmission band (at a position where the frequency is highest).
The SP signal is a signal modulated by BPSK and, as shown in FIG. 7, arranged in such a way that it is inserted once in every 12 carriers in the sense of frequency and once in every 4 symbols in the sense of symbol. The SP signal is used to estimate the characteristics of the transmission path when the receiver side equalizes the waveform. Therefore it is inserted only for synchronous modulation involving waveform equalization (QPSK, 16QAM, 64QAM).
The TMCC signal and the AC signal are signals modulated also by BPSK and arranged at respective positions in each symbol as shown in FIGS. 8 and 9. FIG. 8 shows their positions in an OFDM frame for differential modulation and FIG. 9 shows their positions in an OFDM frame for synchronous modulation. The AC signal is used for the transmission of additional information, while the TMCC signal is used for the transmission of transmission control information.
The TMCC signal carries 204-bit (B0 through B203) information that is completely contained in a unit of OFDM frame. FIG. 10 shows the contents of information assigned to a TMCC signal.
Bit B0 is a bit to which the signal operating as reference for amplitude and phase of differential modulation is assigned.
Bits B1 through B16 are bits to which a sync code (synchronizing signal) to be inverted on a frame by frame basis is assigned. The receiver detects the bit pattern of the sync code to detect the synchronization of the TMCC signal and that of the OFDM frame.
Bits B17 through B19 are bits to which the identification signal of the segment for identifying the frame as one for synchronous modulation or for differential modulation is assigned.
Bits B20 through B121 are bits to which TMCC information (of 120 bits) is assigned. The TMCC information describes the carrier modulation mode of the information signal, the convolutional coding ratio, the interleave length, the number of segments and so on.
Bits B122 through B203 are bits to which parity bits are assigned.
The transmitter generates the OFDM frame in the frame configuring section of the channel encoder. The receiver firstly establishes the synchronism of the symbols on a symbol by symbol basis and performs an operation of FFT. Subsequently, it detects the synchronizing signal described in the TMCC signal and establishes the synchronism of the frames to decode the data contained therein.
When the receiver shifts from a channel to another, it selects the oscillation frequency of the local oscillator for another time to restart the operation of receiving an RF signal. Therefore, when shifting the channel, the receiver has to carry out for another time the operation of establishing the synchronism of the symbols on a symbol by symbol basis, performing an operation of FFT, subsequently detecting the sync code described in the TMCC signal and then establishing the synchronism of the frames to decode the data contained therein.
However, when detecting the synchronism of frames, it is necessary to detect the sync codes of at least two frames. This means that a time span longer than that of a frame period has to be spent for the detection. For instance, according to the ISDB-T Standard, the frame length of an OFDM frame is about 250 ms at most. Then, about 250 ms has to be spent to detect the synchronism of frames. If the synchronism of frames is not detected, it is no longer possible to extract the SP signal that defines the positional arrangement of frames and data on a frame by frame basis, decode the TMCC signal and identify the switching position of puncturing. Then, no data can be output. In short, conventionally a very large amount of time has to be spent for a switching operation from the time of a channel shift to the that of outputting audio and video data after the shift.
This problem arises regardless if connected transmission is used or not.